1. Field of the Invention
This invention relates generally to processes and apparatuses for shaping workpieces by means of electrical discharges, and more particularly to a method and apparatus for shaping a workpiece by impressing intermittent voltage pulses across a working gap between a workpiece and an electrode and controlling the lengths of the voltage pulses in accordance with the condition of said working gap.
2. Description of the Prior Art
In general, the condition existing in the working gap of a process for shaping or etching a workpiece by means of an electrical discharge may change rapidly causing damage to the electrode as well as the workpiece. For example, powdered waste material from the shaping operation may accumulate at a point in the working gap, causing the current density at that point to be increased, and resulting in the production of a relatively large cavity on the surface of the workpiece. In the prior art, it was customary to have an operator observe the condition existing in the working gap and to control the application of electrical voltage or current in response to his observations. However, controlling the shaping process in this manner proved to be an extremely difficult task which could be accurately accomplished only by extremely skilled operators.
In order to explain in greater detail the problems created when conventional processes and apparatuses are used, attention is directed to FIGS. 1 thru 4 which illustrate a prior art apparatus and its operation.
FIG. 1 illustrates an electrical discharge shaping electrode 1 positioned adjacent a workpiece 2. The electrode 1 and the workpiece 2 may be immersed in an insulating fluid such as oil or kerosene. The working gap, which is the space between the electrode and the workpiece, is therefore filled with whatever insulating fluid is used. A plurality of transistors 3A, 3B . . . 3N, which are connected in parallel and coupled to the workpiece 2, are provided to generate square wave voltage pulses to create intermittent discharge currents in the working gap. As will be understood by those skilled in the art, the number of transistors used depends upon the amount of discharge current required. For example, only one transistor is needed when a very low discharge current is required, while a large number of transistors are required for high discharge currents. A plurality of resistors 4A, 4B . . . 4N are coupled to the collector electrodes of resistors 3A, 3B . . . 3N, respectively, for controlling and balancing the current flow through the transistors. A second plurality of resistors 5A, 5B . . . 5N coupled to the base electrodes of resistors 3A, 3B . . . 3N, respectively, are provided to control the base current of the transistors. A timer 6 is provided which includes a pulse generating circuit comprised, for example, of a non-stable multivibrator, a monostable multivibrator, or a flip-flop circuit. A pulse amplifier 7 is coupled to timer 6 at its input and to resistors 5A, 5B . . . 5N at its output to supply switching pulses from timer 6 to transistos 3A, 3B . . . 3N. A DC power source E.sub.o is coupled between shaping electrode 1 and the respective emitter electrodes of transistors 3A, 3B . . . 3N.
FIGS. 2A and 3A illustrate various voltage waveforms which may exist in the working gap of the apparatus of FIG. 1, while FIGS. 2B and 3B illustrate various current waveforms which may exist in the working gap of the apparatus of FIG. 1. In FIGS. 2A, 2B, 3A and 3B, the pulse length 8, quiescent period duration 9, no-load voltage impressing time 10, discharge duration 11, no-load voltage 12, discharge voltage 13, discharge current 14, peak discharge current 15, and average processing current 16 are illustrated. In FIGS. 2A and 2B, the pulse duration and quiescent period duration are maintained constant, while in FIGS. 3A and 3B, the duration of the discharge in the working gap is maintained constant. FIGS. 2A, 2B, 3A and 3B illustrate the operation of two conventional types of electrical workpiece shaping apparatuses under ideal conditions; that is, when no waste FIG. 2A, has accumulated in the working gaps of these apparatuses. As can be seen from FIGS. 2A and 3A, under ideal conditions, the no-load voltage 12 appears during each operating cycle and the average no-load voltage impressing time 10 is consequently controlled, since the average 2A voltage within the working gap is maintained constant. However, as waste powder accumulates in the working gap, the no-load voltage impressing time 10 is diminished, and may eventually vanish, resulting in damage to the workpiece, as described above. This situation is illustrated in FIG. 4A, wherein the voltage pulses are illustrated as remaining at the discharge voltage level 13, and never rise to the no-load voltage level 12. In physical terms, this means that a discharge occurs in the working gap at the instant transistors 3A, 3B . . . 3N are switched on. As a result of this condition, the average discharge duration and the average discharge current are substantially increased, as shown in FIG. 4B. Thus, the average processing current level 16 is at a higher level in FIG. 4B than in either FIG. 2B or FIG. 3B. If this unfavorable condition exists in the working gap for a period of time, the electrical discharge which may be concentrated at a point, causes further deterioration in the condition of the working gap, further increasing the possibility of damage to the workpiece.
In order to overcome the condition illustrated in FIG. 4, it is necessary to decrease the average processing current and to return the working gap to its normal condition. It is possible to decrease the average processing current by detecting the average condition of the working gap by measuring the average no-load voltage impressing time, for example, and regulating the duration of the discharge in the working gap in response to the average condition detected. However, in conventional processes using conventional apparatuses, it is difficult to rapidly follow the changing conditions of the gap, and therefore it has been difficult to properly respond to the changing conditions.